1. Field of the Invention
The present invention relates to technology for making mask data, and particularly to a system, a method and a program for generating mask data, exposure mask and semiconductor device in consideration of optical proximity effects
2. Description of the Related Art
With the miniaturization of a large scale integrated circuit (LSI), an LSI having a minimum line width equal to or shorter than a half of the wavelength of a light source of exposure equipment is required. Due to the miniaturization of such an LSI, a problem arises because of optical proximity effects (OPE) where a pattern with a desired shape cannot be transferred onto a wafer due to the influence of an adjacent pattern on the image of a mask pattern projected on the wafer. Hereinafter, the amount of change in shape of an actual pattern formed on the wafer, as compared to the desired pattern, is referred to as a “pattern change amount”.
Optical proximity correction (OPC) has recently been made where a correction of a mask pattern is made in consideration of the optical proximity effects in the photolithography process. Also, process proximity correction (PPC) has been made where correction of a mask pattern is made in consideration of the proximity effects in the etching process and the like in addition to the optical proximity effects. For example, the following method for generating mask pattern correction data has been proposed. The method includes forming a pattern on a wafer by applying an appropriate mask pattern to a process used in the manufacture of a semiconductor device; generating a prediction model of a pattern in consideration of the pattern change amount by measuring the shape and dimension of the formed pattern; predicting the finished shape of a circuit pattern of a semiconductor device with the prediction model; and generating mask pattern correction data based on the predicted finished shape.
When generating the prediction model, it is required to set an area, including patterns influencing each other, on the dimension or shape of a prediction target pattern. Then, the set area is assumed to be a reference area, and the mask pattern is corrected based on the prediction model included in the reference area. The “reference area” is an area for referring to the mask pattern correction data centering on a focused point in order to determine the correction amount to be applied in the process proximity correction centering on the focused point of a given pattern. In general, the reference area is often set in a manner by including a plurality of patterns exercising optical influence on each other, and the size of the reference area is set within two microns.
However, due to the influence of the pattern density in the surroundings, a phenomenon that the pattern change amount in an area where the process proximity correction is made is different is often observed. This phenomenon is, for example, termed “stray light” or “flare”, which is generated by non-uniformity of optical elements and the like used for an optical system of an exposure system, or by a deviation from a desired shape. Alternatively, the phenomenon is referred to as loading effects produced in the resist development process or the etching process. Here, the above-mentioned phenomenon is hereinafter described as a “flare” phenomenon. The “pattern density” (or “pattern density”) is a value found in a manner such that the space for a pattern in a given area is divided by the space of the given area. The pattern density in the mask pattern is referred to as the “mask pattern density”. There is a case where the reference area is set to include surrounding areas which are a cause of the flare phenomenon and where the prediction model is generated. The space of the reference area becomes large due to consideration of the flare phenomenon.
In the LSI including a memory such as a random access memory (RAM), the same design pattern is repeatedly used. Therefore, design data, which is compressed while the same design pattern is divided into the graphic and arrangement information, often has a layered structure. When the space of the reference area is increased, there is a case where all design data of a RAM and the like are included in one reference area in an LSI including a memory. In such case, the memory cell patterns in the central portion of a memory cell array and in the peripheral portion of the memory cell array may have a differently influenced by the process proximity effects due to the difference in respective pattern density, even if they are the same memory cell patterns. Hence, it is not possible to apply the same correction amounts to all memory cell patterns. Thus, after developing the layered structure of the design data (hereinafter, referred to as a “flattening of design data”), a process proximity correction for each memory cell pattern is required. Therefore, if the process proximity correction in which a wide reference area is set in consideration of the flare phenomenon, the amount of mask data after the process proximity correction increases due to the flattening of the design data, and thus the time required for the process proximity correction increases.
As a method of the process proximity correction in consideration of problems following the flattening of the design data, there is a method in which the process proximity correction is first made in a small reference area and the process proximity correction is added in the electron beam exposure process together with an electron beam irradiation amount correction.
However, since the process proximity correction is made together with the electron beam irradiation amount correction, as performed in electron beam equipment in the above-mentioned method, it is difficult to check the process proximity amount correction applied to a manufactured semiconductor device. The check referred to here is to check whether or not a circuit pattern of a semiconductor device manufactured by use of a generated mask pattern based on an applied correction amount or correction shape is completed as desired, by using design data, post-correction mask pattern data and a prediction model. Furthermore, the above-mentioned check is to check whether or not the circuit pattern can be formed in a range of a predetermined dimension change amount, or whether or not the circuit pattern can be formed in a range of a predetermined dimension change amount under tolerable process changes (for example, an exposure amount change, a focus change, a mask pattern dimension change, a resist sensitivity change, a development time, a temperature change, an etching condition change, and the like).
Moreover, if the correction target area of the electron beam irradiation amount correction fails to agree with the reference area of the process proximity correction, the accuracy of the process proximity correction is not sufficient. There is a known case where an area of several microns to several hundred microns is required as the reference area of the process proximity correction, while a common electron beam irradiation correction target area is of several tens of microns at a dimension on a mask.